Wouldn't it be fair if they are able to live a good life and only have to work 40 hours a week?

For some people that's certainly good enough and they don't ask for more. If you feel that way than this argument just might not appeal to you, but consider that we're speaking of a totally hypothetical world where people who make shirts also make $20/hr. That's not the case, the people who make shirts and electronics and food and most other things are lucky to make $20/month or anywhere close to the standard of living you'd feel comfortable with. That's the reality of capitalism for most people, and it has to be that way in order to maintain any semblance of balance in the global economy.

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To go in a different direction tho the problems in capitalism are greater than your exploitation as an individual. There is a clash of interests inherent in capitalism. All production and nearly all economic activity of any magnitude is done in order to turn profit. Everything in the world is subject to that 'drive' and in practice it's very destructive. This is mostly done across the world by the brutal exploitation of people and the environment.

Why do we depose foreign governments? To secure our economic interests over theirs. Why do we dump chemicals into the river? Why don't we take every precaution possible to prevent oil spills? It's cheaper not to. Why do we keep people in slave-like conditions (or literal slavery) to mine lithium and harvest cocoa? Because we demand a lot of it in order to keep our own wheels turning. Why do you have to risk your life and livelihood to get an education? Because every economic activity has to be preformed in the pursuit of capital, rather than in pursuit of the subject you're really interested in.

These problems are shitty enough on a case by case basis, but when you put them in a boiling pot with everything else it's a pretty insurmountable problem that's destroying the world, killing a lot of people, and making a lot more suffer to one degree or another. All boils back down to the same thing. None of this would happen if it wasn't rational in the scope of our economic system.